Conventionally, there is known a vehicle charging system configured to supply a battery device installed in a vehicle with electric power from a power supply in a general household by connecting a receptacle and a charging port via a charging cable. For example, the receptacle is installed in a house and connected to a commercial power supply, and the charging port is provided in a vehicle such as an electric car or the like. For example, Japanese Patent Application Publication Number 2010-22163 (hereinafter referred to as “Document 1”) discloses such a vehicle charging system.
A vehicle charging system described in Document 1 includes a charging cable, and a charging control device configured to control charging of a battery device, which a vehicle is equipped with, through the charging cable. The charging cable includes a power cable for supplying the battery device with electric power from an external power supply, and a signal generation circuit configured to generate a control signal (a pilot signal) corresponding to a current capacity to be supplied to the vehicle. The charging cable also includes a control line for transmitting a control signal to the vehicle, and a power supply control unit configured to perform power supply control based on a voltage level of the control line.
The charging control device includes a charging control unit and a status detection unit. The charging control unit is configured to control charging of the battery device based on a control signal if the charging cable is connected to the vehicle. The status detection unit is configured to detect malfunction (a defect) of the charging cable based on a signal level of the control line.
The vehicle charging system is configured, based on a pilot signal transmitted through the control line, to determine a connection status between the external power supply and the charging cable, and also determine whether or not to allow electric power from the external power supply to be supplied to the vehicle. This communication function (a CPLT function) between the charging cable and the vehicle is in conformity with SAEJ1772:2010 that belongs to SAE (Society of Automotive Engineers) standard. In this standard, the pilot signal is set to, for example, a zero voltage (0 V) while the external power supply is cut off.
In the vehicle charging system, the pilot signal is a zero voltage when the control line for the pilot signal is disconnected, but Document 1 is silent about changing the pilot signal to a zero voltage when the control line is not disconnected.
The conventional signal generation circuit included in the charging cable just outputs a pilot signal with a specified constant voltage, or oscillates a pilot signal so as to be a pulse train of specified voltage and frequency. In order to output a zero voltage from the conventional signal generation circuit, an output end of the signal generation circuit therefore needs to be connected to ground (earth) through a relay which is provided between the output end of the signal generation circuit and ground. This configuration has a problem of an increase in cost of the signal generation circuit because a comparatively expensive relay is additionally required.